Process for enameling aluminumrich alloys



highly moisture-laden atmospheres.

Patented Mar. 6, 1951 UNITED STATES PROCESS FOR ENAMELING ALUMINUM- RICHALLOYS Alden J. Deyrup, Westfield, N. J., and James H. Peterson,Wilmington, Del., assignors to E. I. du Pont de Nemours & Company,Wilmington, Del., a corporation of Delaware No Drawing. Application July1, 1947, Serial No. 758,470

4 Claims. 1

This invention relates to the enameling of aluminum-rich alloys, 1 e.,alloys containing at least 80% by weight of aluminum. More particularly,this invention relates to the chemical and heat treatment ofaluminum-rich alloy structures followed by the application and firing ofa vitreous enamel on the treated surface of the structures.

In the copending application of Alden J. Deyrup, Serial No. 635,765,filed December 18, 1945, now Patent No. 2,467,114 issued April 12, 1949,there are pointed out the difficulties entailed in the enameling ofaluminum and aluminum-rich alloy structures with a vitreous enamel. Theabove-said copending application relates to a new enamel compositionwhich is particularly suitable for the enameling of aluminum andaluminum-rich alloy structures;

Although these new enamels have been highly successful in the enamelingof aluminum-containing structures, in the presence of water ormoisture-laden atmospheres the vitreous enamels do not adhere sotenaciously to so-called commercial aluminum-rich alloys as tocommercial aluminum, i. e., substantially pure aluminum. Commercialaluminum contains 99% plus aluminum, whereas commercial aluminum-richalloys contain from slightly greater than 1% to 20% of particularalloying ingredients, for example, manganese, magnesium, copper,chromium, zinc and silicon. The somewhat less tenacious adherence ofvitreous enamel to aluminum-rich alloys has sometimes proven to betroublesome, particularly where the enameled aluminum is exposed forlong periods of time to weathering or This results, not always butsporadically, in a phenomenon referred to as water-spalling, which isevidenced by a flaking-off of the enamel in tiny flakes exposing baremetal. Water-spalling starts at exposed metal-enamel interfaces, forexample, at the edges of structures or atpinholes or other, normallyimmaterial, slight flaws in the enamel coating. Water-spalling mayprogress to minor failure or even to complete failure if subjected tolong periods of exposure. Water-spalling may be a slow process andsometimes occurs only after weeks or months of exposure to tap water orweathering.

It is an object of this invention to provide vitreous enamel coatedaluminum-rich alloys having an improved resistance to water-spalling.

It is another object of this invention to produce a vitreous enamelcoated, aluminum-rich alloy which is free from water-spalling. I

It is still another obejctof this invention to provide a process for thechemical treatment of alun'1inum-richalloy structures and subsequentlyenameling the same with a vitreous enamel whereby to produce a productwhich is much less subject to, or free from, water-spalling.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in general, bypre-treating the surface to be enameled of an aluminum-rich alloystructure with an auqeous alkaline solution of a chromate followed by aheat-treatment at a temperature between 350 C. and the temperature atwhich the aluminum alloy structure is deleteriously af- 'fected by heat,usually about'600 (3., and then enameling the so-treated surface with avitreous enamel.

The aqueous alkaline chromate solution may have substantially any degreeof alkalinity between pH 8.0 and pH 14. The desired degree of alkalinitymay be imparted to the solution in any desired manner, for example, bythe addition of alkali metal hydroxides or by the addition of salts,such as alkali metal carbonates or bicarbonates, which upon hydrolysisproduce alkaline conditions.

Any chromate, including dichromates, which is soluble in water may beused for the above-said pre-treatment since it appears that the chromateions in the solution with which the alloy is treated impart the improvedadherence of the enamel to the treated surface. Alkali metal chromatesare preferred. Dichromates, when added to the alkaline solution, will beconverted to chromates.

The chromate content may Vary from 0.25% to 25%, expressed as potassiumchromate, i. e., the chomate should be present in the solution in aconcentration equivalent in ion content to a concentration of 0.25% to25% potassium chromate.

The aluminum alloy structures need not generally be given a particularcleaning treatment before treatment with the chromate solution; however,if the surface of the alloy is very greasy it may be cleaned with greasesolvents such as trichlorethylene, or the-like.

The chromate solution may be applied to the surface of the alloy to beenameled in any desired manner however, it is preferred to immerse thealloy structure in a bath of said chromate solution. The chromatesolution may vary in temperature from room temperature (about 20 0.). upto the boiling temperature of the solution (about C.). For convenienceof operation, temperatures below room temperature are usually notemployed. The time of treatment may vary from one to thirty minutes.After the treatment with chromate solution, it is preferred to rinse thealloy structure with water to remove any adhering chromate solutiontherefrom.

The above-said chromate solution treatmentof aluminum-rich alloysimproves the adherence to aluminum of any vitreous enamel having amelting point below about 580 C. Best results have, however, beenobtained by the use of enamels of the type disclosed in copendingapplication Serial No. 635,765, above-mentioned. Such enamels contain:

10 to 18 mole percent PbO 38 to 65 mole per cent SiOz 5 to 12 mole percent Li2O to 22 moles per cent NazO 0 to 20 mole per cent K20, and 0 to11 mole per cent TiOz the total alkali metal oxide content(LizO-i-NazO-l-KzO) of the enamel bein between 25 and 36 mole per cent,and the ratio of the sum of said silica content and twice the saidtitanium oxide content of the enamel to the total alkali metal oxidecontent of the enamel being between 1.8 and 3.0, all of said oxidesbeing melted into the enamel composition.

In order to obtain best results in adhesion of the enamel to thechromate-treated surfaces, the enamel should be fired at a temperaturebetween 480 C. and 580 C., and in some cases preferably between 480 C.and 510 C. for a period of between two and thirty minutes. This appliesto both ground and cover coats in case two enamel coats are applied.

It is essential that the alloy, after the chromate solution treatmentand before enameling, be given a heat-treatment at a temperature between350 C. and the heat tolerance limit of the alloy. Preferably, this heattreatment is carried out at a temperature of between 400 C. and 550 C.Such heat treatment is necessary to obtain adequate adherence of enamelto the alloy. The period of heat-treatment may vary from two to tenminutes. The heat-treatment may be carried out in an oven at atmosphericconditions.

The optimum conditions of chromate content in the alkaline chromatesolution, time and temperature of treatment in the chromate solution,and firing time and temperature of the enamel will vary, dependent uponthe composition of the alloy being treated. For example, in the case ofthe pre-enameling chromate treatment of aluminum alloy 61, containingabout 0.25% copper, 0.6% silicon, 1.0% magnesium, and 0.25% chromium,with a bath containing four parts by weight sodium hydroxide, 20 partsby weight potassium chromate and the remainder water, it

is preferred to treat the same for five minutes with the temperature ofthe bath at 50 C. In treatin aluminum alloy 43, containing about siliconwith the same chromate bath composition, it is preferred to treat it forfive minutes with the bath heated at 40 C. When treating aluminum alloy3, containing about 1.2% manganese, with the same bath, it is preferredto treat it for fifteen minutes at a temperature of 40 0. Any of theabove three aluminum alloys treated in accordance with the above-saidpreat a temperature of between 350 C. and the heat tolerance limit ofthe alloy before enameling.

In the case of any particular aluminum alloy, it can readily bedetermined whether or not sufficient enamel adherence to insure againstwaterspalling is obtained by any prescribed course of treatment. Thiscan be determined by means of the following accelerated test whichcorrelates with water exposure results which may take weeks or months.This accelerated test consists in chromate-treating and enameling piecesof the particular alloy to be tested, and then exposing the enamel-metalinterface by gouging, filing, or scratching the same, and immersing thetest sample in 5% aqueous ammonium chloride solution for 96 hours atroom temperature. If no failure occurs back one-eighth inch from theedge of the exposed interface, excellent performance in water exposurewill result from the chromate treatment and enameling conditions used.The particular optimum treatment and enameling conditions for a specificalloy can thereby be established. Once optimum treatment conditions fora given alloy have been established and a chromate bath is used forcontinued treatment of the alloy structures or articles, it will, ofcourse, be desirable to periodically analyze the treating bath and keepthe chromate content and alkalinity of the bath within a few per cent oftheir initial value.

The following examples are given to illustrate certain preferredembodiments of the present invention, it being understood that thedetails set forth in these examples are not to be considered aslimitativar Example I Parts by weight Sodium hydroxide 4 Potassiumchromate Water 76 and having a temperature of 50 C. After five minutes,the sheet is removed, rinsed in tap water, dried, and heat treated at atemperature of about 500 C. for five minutes. The heattreated sheet,after cooling, is then dipped into a water slurry of an enamelconsisting of 38.1% PbO, 25.3% SiOz, 2.2% LizO, 10.2% NazO, 8.3% K20,8.7% TiOz, 2.9% SbzOs, and 4.3% B203. To the water slurry is also added3% of boric acid and 0.5% of potassium chromate, based on the weight ofthe water in the slurry for the purpose of obtaining better enameling.The liquid and solid contents of the slurry are mixed in suchproportions that the sheet, after dipping in the slurry and draining,will contain between 15 and grams (based on dry weight of enamel) persquare foot for each coating; several enamel coatings, which may containpig- 'ments or opacifying agents, with separate firings being applied ifdesired. The enamel is fired at a temperature between 480 C. and 515 C.

After firin and cooling, the enamel-coated structure is gouged throughthe enamel coating and is then placed for 96 hours in a 5% aqueoussolution of ammonium chloride. The coated structure showed noperceptible water-spelling.

Example II Aluminum alloy castings compound of alumi- No. 1 No. 2

P1118 by Parts by weigh weight Water 89. 90. 0 Potassium chromate 2. 00.5 Sodium bicarbonate 2.0 2.5 Sodium carbonate 7.0 7.0

The temperature of the bath and the treatment period are adjusted forany specific alloy treated within the above-mentioned temperature rangesand treatment periods to obtain optimum results.

Successful enameling with subsequent high resistance to failure of bondunder prolonged water exposure may be readily obtained by adheringgenerally to the following principles:

In variation of time of treatment, treatments of too short durationcause incomplete protection against water-spalling. This may berecognized by enameling test pieces and subjecting to the acceleratedwater-spalling test above. Treatments of too long duration result inconsiderable etching. This may result in a loosely adherent depositwhich prevents the enamel from adhering, so that it comes off locally orgenerally during or shortly after cooling. It is practical to determinetimes of treatment, if

preferred conditions are departed from, at which these two defects beginto occur, and then select a time midway between for practical use.

In variation of temperature of treatment, it is found that highertemperatures require shorter times. Speedier operation may thus besecured by appropriate temperature increase, if desired.

In variation of alkalinity, it is found that more alkaline baths causemore rapid treatment. It is desirable to avoid too rapid treatment sothat time of treatment ma be practically controlled at its optimalduration.

In variation of chromate content, increase of chromate content generallygives more flexibility A in time duration of treatment with favorablere- 5 sults, but decreases the speed of action.

Aqueous alkaline chromate solutions have been used heretofore fortreatment of aluminum to produce desired surface appearance and toimprove corrosion resistance. Furthermore, the addition of even verylarge amounts of watersoluble chromates to enamel slurries and slips iswithout effect in preventing water-spalling. The 'reason why treatmentwith chromates and heattreatment prior to enameling in accordanc withthis invention will prevent subsequent waterspalling of the enamelcoating is unknown. It has, however, been definitely established thatthefinal enameled alloy contains a chromium compound between the alloy andthe enamel.

Reference in the specification and claims to parts, proportions andpercentages, unless otherwise specified, refers to parts, proportionsand percentages by weight.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to said details except as set forth in the appended claims.

What is claimed is:

l. The process of producing vitreous enamel coated aluminum-rich alloyshaving an improved enamel bond which comprises placing the alloy surfaceto be enameled in contact with an aqueous alkaline solution containingbetween 0.25% and 25% of a water-soluble chromate for a period ofbetween 1 and 30 minutes, heat-treating the chromate-treated alloy at atemperature between 350 C. and the heat tolerance limit of the alloy fora period of 2 to minutes, and then enameling the so-treated surface witha vitreous enamel having a melting point below about 580 C. by applyingsaid enamel to said heat-treated surface and firing the same above thefusing point of the enamel at a temperature between 480 C.

and 580 C.

2. The process of producing vitreous enamel coated aluminum-rich alloyshaving an improved enamel bond which comprises placing the alloy surfaceto be enameled in contact with an aqueous alkaline solution containingbetween 0.25% and of a water-soluble chromate and having a temperaturebetween C. and the boiling temperature thereof for a period of between 1and 30 minutes, heat-treating the chromatetreated alloy at a temperaturebetween 350 C. and the heat tolerance limit of the allo for a period of2 to 10 minutes, and then enameling the so-treated surface with avitreous enamel having a melting point below about 580 C. by applyingsaid enamel to said heat-treated surface and fusing the same above thefusing point of the enamel at a temperature between 480 C. and 580 C.

3. The process of producing vitreous enamel coated aluminum-rich alloyshaving an improved enamel bond which comprises placing the alloy surfaceto be enameled in contact with an aqueous alkaline solution containingbetween 0.25% and 25% of a water-soluble chromate and having atemperature between 20 C. and the boiling temperature thereof for aperiod of one to thirty minutes, heat-treating the chromate-treatedalloy at a temperature of 400 C. to 550 C. for a period of 2 to 10minutes, and then enameling the so-treated surface with a vitreousenamel having a melting point below 510 C., the enamel being fired at atemperature between 480 C. and 510 C.

4. The process as defined in claim 2 in which the water-soluble chromateis an alkali metal chromate.

' ALDEN J. DEYRUP.

JAMES H. PETERSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,230,958 Warga June 26, 19171,971,240 Tosterud Aug. 21, 1934 FOREIGN PATENTS Number Country Date273,956 Great Britain July 14, 1927 463,790 Great Britain Mar. 30, 1937

1. THE PROCESS OF PRODUCING VITREOUS ENAMEL COATED ALUMINUM-RICH ALLOYSHAVING AN IMPROVED ENAMEL BOND WHICH COMPRISES PLACING THE ALLOY SURFACETO BE ENAMELED IN CONTACT WITH AN AQUEOUS ALKALINE SOLUTION CONTAININGBETWEEN 0.25% AND 25% OF A WATER-SOLUBLE CHROMATE FOR A PERIOD OFBETWEEN 1 AND 30 MINUTES, HEAT-TREATING THE CHROMATE-TREATED ALLOY AT ATEMPERATURE BETWEEN 350* C. AND THE HEAT TOLERANCE LIMIT OF THE ALLOYFOR A PERIOD OF 2 TO 10 MINUTES, AND THEN ENAMELING THE SO-TREATEDSURFACE WITH A VITREOUS ENAMEL HAVING A MELTING POINT BELOW ABOUT 580*C. BY APPLYING SAID ENAMEL TO SAID HEAT-TREATED SURFACE AND FIRING THESAME ABOVE THE FUSING POINT OF THE ENAMEL AT A TEMPERATURE BETWEEN 480*C. AND 580* C.